1. Field of the Invention
The present invention relates to an aluminum alloy casting obtained by cooling and solidifying a melt of an aluminum alloy (an Al alloy), a method for producing the aluminum alloy casting, and an apparatus and a method for producing a slide member from a metal melt.
2. Description of the Related Art
In most of internal combustion engines, a cylindrical slide member (a sleeve) is inserted into a bore formed in a cylinder block, and a piston is reciprocated in the sleeve. When the piston is slidably in direct contact with the inner wall of the bore in the cylinder block, the inner wall may be abraded. The sleeve functions to prevent the abrasion of the inner wall.
When the cylinder block is produced by a casting method, the sleeve is disposed in a predetermined position in a cavity, and then a melt for forming the cylinder block is introduced to the cavity, whereby the sleeve is surrounded by the melt. Thus, a so-called cast coating (enveloped casting) is carried out to obtain the cylinder block containing the sleeve.
As a material for the sleeve, an Al—Si alloy having a high silicon (Si) content (a high-silicon alloy) is generally used because the alloy is lightweight, highly abrasion-resistant, and highly strong. However, the sleeve composed of the high-silicon alloy is not suitable for the cast coating with the melt for the cylinder block, whereby it is difficult to obtain a sufficient bond strength between the sleeve and the cylinder block.
The problem can be solved by using the high-silicon alloy also in the melt for the cylinder block. However, the high-silicon alloy is generally expensive, and thus this method is high in cost.
The above problem can be solved also by using in the sleeve an Al alloy such as an Al—Fe—Mn—Si alloy, which is suitable for the cast coating with respect to the cylinder block and is excellent in abrasion resistance.
However, when a melt of the Al—Fe—Mn—Si alloy is cast to produce the sleeve, the resultant casting (the sleeve) contains a needle-shaped coarse crystallized substance of an iron-based (Fe-based) intermetallic compound. The needle-shaped coarse crystallized substance can cause fracture, whereby the obtained sleeve cannot be sufficient in strength and toughness.
From this viewpoint, several studies have been made on miniaturization of the crystallized substance. For example, Japanese Laid-Open Patent Publication No. 2007-216239 discloses a technology containing the steps of ultrasonically vibrating the melt before the melt is cooled below the liquidus-line temperature (the solidification starting point), and then solidifying the melt.
In the case of using such ultrasonic vibration (at a frequency of 20 kHz or more) as described in the conventional technology of Japanese Laid-Open Patent Publication No. 2007-216239, though a large number of embryos can be generated, it is difficult to apply an energy sufficient for growing the embryos to crystal nuclei. Therefore, most of the embryos are remelted, whereby needle-shaped crystals of the Fe-based intermetallic compound are generated as shown in FIG. 9 of Japanese Laid-Open Patent Publication No. 2007-216239. As is clear from this, the conventional technology described in Japanese Laid-Open Patent Publication No. 2007-216239 is disadvantageous in that it is difficult to prevent the generation of the needle-shaped crystallized substance, which can cause fracture.
The applicant has proposed, in Japanese Laid-Open Patent Publication No. 2008-155271, a technology of vibrating the melt at a frequency of 1000 Hz or less when the temperature of the melt is higher than the solidification starting point but is lower than a temperature 10° C. higher than the solidification starting point.
By using the technology described in Japanese Laid-Open Patent Publication No. 2008-155271, the miniaturization of the crystallized substance can be achieved while reducing the generation of the needle-shaped crystal. Still there is a demand for further miniaturization.
The sleeve for the cylinder block can be produced by various methods. For example, a sleeve composed of an iron-based material is generally produced by a spin casting method. In this method, since the iron is relatively heavy, a large production apparatus may be required.
A sleeve composed of an Al alloy can be produced by a spray forming method or the like as a conventional technology described in Japanese Laid-Open Patent Publication No. 2000-109944. In this technology, a final extrusion process is needed to obtain the sleeve material.
Furthermore, a casting excellent in mechanical properties such as abrasion resistance can be produced by utilizing, for example, a centrifugal force for arranging hard metal compound grains on an outer surface of the casting (see Japanese Laid-Open Patent Publication No. 58-116968).
In the technology described in Japanese Laid-Open Patent Publication No. 2000-109944, higher cost, time, and effort may be required to produce the Al alloy sleeve because the final extrusion process is required.
The technology described in Japanese Laid-Open Patent Publication No. 58-116968 is designed only to improve the abrasion resistance of the outer circumferential surface of the casting, and the obtained slide member has only limited application. Thus, the casting cannot be used as the sleeve for the cylinder block, etc.
In the conventional technologies described in Japanese Laid-Open Patent Publication Nos. 2007-216239 and 2008-155271, the sleeve casting can be produced with improved mechanical properties by vibrating the Al alloy melt to miniaturize the cast metal structure. However, to use the casting as a slide member, the sliding surface of the casting should be excellent in abrasion resistance.